Sertraline-weight gainer!?

Has anyone else experienced this as a side-effect? I have been on these since may this year and since then I cannot lose the weight! The lowest I have been down is 11'2 but it wont go any further. I diet, go to a diet class and exercise several times a week e.g. walking up Bennachie, kick boxing, Kettlebell classes, curves so its not for the want of trying! I am exactly the same weight as when I started on Sertraline but I want to get under that weight. any help would be greatly received.